You can just write the private key in a letter, and seal the letter and gave it to a lawyer who will give it to your children when you die.
there is no way I would trust a lawyer with this, he opens letter and burns it after. No one would ever know!
Split the private key using Samir's Secret Sharing algorithm and give several trusted parties a fragment. For example, a 3-of-5 scheme where you give a fragment to a spouse, friend, family member, lawyer and bank deposit box. Any 3 fragments can be used to recover the private key. A short instruction on what Bitcoin is and how to use the private key you can spread more liberally to your beneficiaries.
I was actually thinking recently - this would be an interesting place for John K to branch out, and maybe finally form a proper company, since he already holds so much in escrow and has trust established unlike any other member of the community. Since he's quite knowledgeable in Bitcoin, he can offer a Q&A session with family, whereas a letter can't do that and you need to have everything either fully explained to them while you and they are both alive, or a pretty darn good (long) explanation in text.
You would pay someone like John to both hold a key fragment and explain to the beneficiaries what Bitcoin is and what their options are. Of course, if you don't go through someone like John, you might not expect them to realize the massive death taxes they'd be liable for in most countries.